Twins are multiplying according to new research from the Centers for Disease Control and Prevention.

In 2009 one out every 30 births was a twin. In 1980 it was one out of 53.

Twins born on Nov. 11, 2011 in a Pakistan hospital. (ASIF HASSAN/AFP/GETTY IMAGES)

The biggest increase came among mothers age 40 and older. They were more likely to use fertility treatments and to have two embryos implanted during in vitro fertilization. About 7 percent of all births for women 40 and older were twins, compared to 5 percent of women in their late 30s and 2 percent of women age 24 or younger.

“You have a double whammy going on. There are more older moms and more widespread use of fertility-enhancing therapies,” co-author Joyce Martin told the Associated Press.

The news, interestingly, comes just as the latest National Geographic issue devotes its cover feature to twins.

In it, writer Peter Miller examines how by studying identical twins, researchers are learning more about the nature vs. nurture debate, particularly as it concerns health issues.

A researcher told Miller that twins have become integral to medicine’s understanding of the importance of DNA. For instance, there’s a 70 percent chance that when one twin is diagnosed with autism, so too will his twin.

“We forget that 50 years ago things like alcoholism and heart disease were thought to be caused entirely by lifestyle. Schizophrenia was thought to be due to poor mothering. Twin studies have allowed us to be more reflective about what people are actually born with and what’s caused by experience,” geneticist Danielle Reed told Miller in the article.

The piece also touches on the question that plagues many of us parents constantly: How much does “parenting” matter?

“When they compared identical twins raised in different families, like the Jim twins, with those raised in the same family, they found each pair’s IQ scores to be similar. It was as if it didn’t matter in which family the twins had been raised. That didn’t imply, [Thomas] Bouchard and his colleagues were quick to point out, that parents have no impact at all on their children. Without a loving and supportive environment, no child can reach his or her full potential, they said. But when it came to explaining why a particular group of children ended up with different IQ scores, 75 percent of the variation was due to genetics, not parenting,” says Miller.

Do you have twins? How much similarity do you see in their behavior? Is it fair to attribute those characteristics to nature?